Evaporator for treating surfaces



July 25, 1944.

c. w. HEWLETT 2,354,521

EVAPORATOR FOR TREATING SURFACES Filed Jan. 7, 1943 2 Sheets-Sheet lInventor: Claret-ice W. Hewlett by W 6. JMZM His Attorn e9 July 25,1944. c, w, HEWLETT 2,354,521

EVAPORATOR FOR TREATING SURFACES Filed Jan. '7. 1943 2 Sheets-Sheet 2Invntor: Clarence W. Hewlett is Attorneg.

- terior surface of the support.

Patented July 25, 1944 EVAPORATOR FOR TREATING SURFACES Clarence W.Hewlett, Schenectady, N. Y., assignor to General Electric Company, acorporation of New York Application January 7, 1943, Serial No.- 471,537

15 Claims.

My invention relates to an improved apparatus for depositing substanceson a surface or surfaces, particularly for depositing vaporizablesubstances on a plurality of surfaces simultaneously.

One object of the present invention is to provide an improved apparatusfor applying substances to surfaces by vaporization, such that thecondensed deposit shall be of uniform thickness throughout its extent onthe surfaces.

A further and particular object of my present invention is to provide animproved apparatus for insuring that the physical nature of thecondensed deposit, that is, its state of crystallization, crystalorientation, compactness-and other characteristics, shall be of a highdegree of uniformity throughout the extent of the deposit.

In my U. s. Patent 2,337,329, patented December 21, 1943, I havedisclosed a means for obtaining by evaporation in a vacuum a condenseddeposit upon a plurality of objects having flat surfaces. In accordancewith the invention described in that prior patent, the objects uponwhich the vaporizable substance is to be deposited are attached to theinterior surfaces of a spherical support in such manner that the flatsurfaces to be coated are all tangent to an assumed single sphericalsurface concentric with the in formed in the bottom of the support anevaporator is fitted having a surface upon which the substance to bevaporized to the objects is placed, and a heater means is provided forthe evaporator surface. The evaporator is so arranged that the surfaceof the substance to be evaporated is flat and of quite small arearelatively to that of the assumed spherical surface to which thesurfaces to be coated are tangent, andthis surface of the substance tobe evaporated is tangent to the assumed spherical surface.

Under proper conditions, by utilizing the apparatus of said priorpatent, the surfaces of the several flat objects all receive deposits ofthe same thickness.

It has been found, however, that when articles are coated intheabove-described apparatus, the physical nature of the depositthroughout the several surfaces to be coated may at times varyundesirably, for the reason, apparently, that the molecular raysproceeding from the evaporating substance which is located at the bottomof the spherical support, strike the several surfaces to be coated atwidely different angles, and a difference in crystal structure of thedeposit on the several surfaces, or a difference of orienta In anaperture tion of the crystal aggregates, or both, may thereby result.

Further, if the surfaces to be coated have been roughened, as isdesirable for certain purposes, the microscopic projections orprotuberances of those of the receiving surfaces upon which themolecular rays make a large angle of incidence may at times tend toshadow or mask the adjacent microscopic valleys or depressions, andthereby to produce microscopic uncoated areas or spots in thedepressions of the roughened surface.

In accordance with my present invention these dimculties are overcomeand the above-mentioned objects, particularly the object of insuringuniformity of the physical nature of the deposit throughout its extent,are attained by providing, in place of the flat-surfaced evaporatorlocated tangent to the spherical surface to which the several receivingsurfaces to be coated are tangent, a spherical evaporator structuremounted within and concentric with the spherical support upon the innersurface of which the objects having the surfaces to be coated byvaporization are mounted.

In order that the uniformity of thickness and uniformity of the physicalnature of the sub stance deposited by evaporation on the objectsdisposed about the interior surface of the spherical support at thecenter of which the evaporating substance is located, may be attained,the surface of the evaporator element from which the molecular rays areprojected should be spherical. For most substances, however, the vaporpressure is not great enough for practical purposes in the process ofdepositing the coating by vaporization except at temperatures abovetheir melting points. In the case of these substances, therefore, it isnot practicable to attack the problem directly by attempting to providea spherical mass of the evaporating substance, as this procedure wouldnecessitate locating at the center of the spherical supporting surfaceupo which the objects are mounted, a spherical globule in liquid form ofthe substance to be evaporated.

On the other hand, if to overcome this latter difiiculty it is proposedthat a container or pan for the substance in liquid form be located atthe center of the spherical support, then those objects mounted in thelower half of the support will receive little or no deposit and thedensity of the deposit on those objects in the upper half of thespherical support will be greatest at the top and will decrease towardthe equatorial To overcome these dimculties in obtaining a sphericalsource of the molecular rays, I preferably provide in the center of thespherical support within which the objects to be coated are mounted anevaporator comprising a spherical shell, perforated for the passage ofthe molecular rays of vaporized substance, enclosing a pan or likeholder or container charged with the vaporizable substance, and providedwith means for supplying heat to maintain elevated temperature and tomelt and to evaporate the substance in the enclosed pan.

The heating means is preferably so arranged as to have no interfering ormasking effect on the molecular rays which are projected radially fromthe outside surface of the evaporator. The construction materials forthe various elements of the evaporator, for any given substance to bevaporized, must have a higher melting point than that of the substance,and must not be attacked by the vapor at its operating temperature. The

perforations through which the vapor passes out from the spherical shellof the evaporator are so arranged that a substantially uniformdistribution of the projected vapor is secured. To insure such a desiredtemperature relationship between the vapor, the evaporator shell, andthe pan or holder together with the vaporizable substance placedtherein, that the vapor will be supplied in suflicient quantity by theevaporator and its passage through the shell perforations will beunimpeded, a heat insulation means is provided associated with the panor holder element.

The novel features which are considered to be characteristic ofmypresent invention are set forth with particularity in the appendedclaims. My invention itself, however, both as to its organization andmethod of operation together with further objects and advantages thereofmay best be understood by reference to the following description takenin connection with the accompanying drawings wherein Fig. 1 illustratesin partial section an apparatus for coating surfaces by vaporizationembodying the present invention; Figs. 2 and 3 are enlarged viewsrespectively of the exterior and interior of spherical evaporatorstructure employed in the embodiment of Fig. 1; Fig. 4 is an enlargedview of heater supporting elements; Fig. 5 is an exploded view on anenlarged scale of a pan or holder element for the substance to bevaporized together with heat shielding means for the holder element; andFigs. 6 and 7 illustrate in side elevation and plan view respectively amodification in which the evaporator structure is other than spherical,for example hemispherical in form.

In Fig. 1 the numeral l0 designates a spherical support the interiorsurface of which is provided with brackets II or like means fordetachably holding in operative position any desired number of objectsl2 such as rectifier electrode elements, for example, having flatsurfaces is to be coated with a deposit produced by vaporization ofsuitable substances such as bismuth and selenium. The elements l2 are somounted with reference to the spherical support III that the flatsurfaces I! are all tangent to an assumed spherical surface concentricwith the interior surface of the spherical support. An opening H, whichmay conveniently be located at the bottom of the spherical support I0,is provided therein, together the shell at an with other openings suchas perforations I! in various parts of the support H).

To permit the performing of the vaporizing process under vacuumconditions a bell jar IE or the like surrounds the spherical support Hiand is joined by a temporary seal H to a floor plate I8. A cylinder orlike member l9 having large openings 20 therein supports the sphericalmember I 0 on the floor plate It. To evacuate the interior of the belljar and the spherical support i0 covered thereby a duct 2| is providedconnected to a vacuum pump (not shown).

In the system disclosed in my above-mentioned prior patent theevaporator, employed in connection with a, spherical support such as illfor vaporizing a substance for the purpose of depositing a coating uponsuch surfaces as l3, comprises a heated fiat-surfaced plate inserted inthe opening ll. The surface of the substance placed on this plate in thevaporizing process was tangent to the assumed spherical surface to whichthe surfaces l3 are tangent. In the" embodiment of my present inventionillustrated in Fig. 1, however, the evaporator, designated generally bythe numeral 22, comprises a spherical shell" having perforations 24therein and mounted concentrically with the interior surface of theencircling spherical support In by a suitable standard 28 extending fromthe floor plate 18 to the evaporator.

As more clearly shown in Figs. 2 and 3, the latter figure being anenlarged interior view partially in transverse section of the enlargedperspective view of Fig. 2, the spherical shell 22 is preferably formedin two hemispherical sections 28 and 21 which are detachably fitted orhinged together. The evaporator encloses a pan or holder 26 (Fig. 5) tobe charged with the substance 28a to be evaporated. The pan 28 ismounted centrally of the spherical shell 23 by means of a pin 29extending downwardly therefrom into a supporting standard or stud 30which is secured to the lower shell section 21.

Various means may be employed for supplying the heat required forvaporizing the substance in pan 2!, for example, by induction, byelectron bombardment, by radiation, etc. I prefer to supply the heat byradiation from a hot filament. The heating means in filament form may bear ranged either outside or inside of the evaporator shell 23. I prefer,however, to place the heating means inside of the shell for greatereconomy of heat required, and further because this inside disposition ofthe heating means entails no shadowing or masking effect on themolecular rays as they are projected from the perforations 24 of theshell.

The heating element preferably takes the form of a bare metallicfilament Si or like element, which may be in coiled form as shown, orotherwise arranged. The heating element, coil 3|, is distributed, asevenly as possible, near the entire inner surface of the shell 22 andthus surrounds the pan or holder 28 which is charged with the substanceto be evaporated.

The heating element may be held in place within the shell by anysuitable means, for example by suitable insulators on frame orsupporting means which may be secured to the spherical shell or to aportion thereof. In the present embodiment of the invention the heaterelement mounting means comprises a pair of frame elements 32 and 33,shown separate from the shell in Fig. 4 for clearness, fitted togetherat right angles to each other by slots 34 and 35 and having their lowerends 36 and 37 shaped to cooperate with pairs of slots 36 and 39 atright angles to each other and formed in a hub or enlarged portion 46 ofthe standard or stud 30 upon which the pan or holder 28 is mounted. Theheating element 3| is secured to the two frame elements 32 and 33 bysuitable insulators 4i.

Two leads 42 and 43, insulated from the spherical shell 23 and passingtherethrough, are connected to a source (not shown) of heating currentfor the element 3|.

. The heating element 3! for the pan 28 thus preferably takes such ageneral form as to be bounded on its outside by an assumed sphericalsurface of smaller diameter than that of the inside surface of theperforated spherical shell 23 and on the inside by an assumed sphericalsurface of larger diameter than that of the pan assembly.

The perforations 24 are preferably countersunk from the inside of theshell 23, as illustrated for example at 44, Fig. 3, so that they becomesharply defined circular apertures, rather than channels, through whichthe vapor escapes from the interior of shell 23 to the region outside ofthe latter shell.

While the number of spaced perforations 24 in the shell 23 required toproduce a substantially uniform distribution of the vapor after it hastraveled a short distance from the evaporator may vary over aconsiderable range, I have determined that when the number is of theorder of ninety-two for a shell 23 of suitable diameter a desiredsubstantial uniformity of vapor distribution is obtained. A largernumber of perforations may be employed but the perforations of necessitywill then be of smaller diameter and more difflcult to produce.

The positions of the perforations 24, a portion of which are indicatedin Figs. 1, 2 and 3, may be determined as follows: (1) locating twentyof them at the vertices of the regular dodecahedron which may be assumedto be inscribed in the shell 23, (2) after laying out these twentyperforations which occupy a pentagonal array on the spherical surface,twelve other perfora tions are located at the center of each sphericalpentagon, (3) each pentagon then is divided into five similar sphericaltriangles and (4) a perforation then is located at the center of each ofthe sixty triangles. While the resulting ninetytwo perforations 24 arenot rigorously uniform in distribution over the surface of shell 23their location is satisfactory for practical operation.

In order that the vapor may escape from all of the equal perforations 24in shell 23 at the same rate it is necessary that the vapor pressurewithin the shell 23 be the same at all points of the interior surface ofthe latter shell. For this reason, the combined area of the perforations24 must be small compared to the unperforated area of the shell 23 andmust also be small compared to the area of the surface of theevaporating substance with which pan 28 is charged. If the latterrequirement is met, the other requirement, that the total perforationarea be small compared to the shell area, is also met. I have found thatsatisfactorily uniform deposits on the objects to be coated are obtainedby so determining the size of the perforations 24 for a given numberthereof that their combined area is of the order of one-ninth that ofthe pan 28.

The pan 28 and the vaporizable substance 28a contained therein must bemaintained at a temperature such that the vapor pressure of thesubstance is high enough to cause the required amount of vapor to escapethrough the perforations 24 in a reasonable time, and at the same timethe shell 23 must be maintained at a temperature high enough to preventthe condensing of vapor on its inner clogging of the perforations. Thetemperature of the shell 23 should therefore be maintained slightlyhigher than that of the substance being vaporized in the pan 28, andthis requirement is achieved in accordance with my invention byproviding the pan 28 with an adequate amount of heat shielding so thatthe heat flow into the pan, at a temperature appreciably below that ofthe shell 23, is equal to the latent heat carried off from the substanceby the evaporation.

For the latter purpose the pan 26 is partially enclosed byheat-shielding or radiation-reducing means comprising, for example, aseries of stacked elements or plates 45, which may be of any suitablematerial, as aluminum for low temperature operation or thin molybdenumfor high temperatures, mounted below the pan, and a similar series ofplates 46 above the pan, these metals providing bright metal reflectingsurfaces aiding materially in the radiation of heat.

The lower shield plates 45 are mounted between the bottom of pan 28 anda base plate 41. Centrally disposed openings 48 (Fig. 5) are formed inthe plates 45 and 41 through which passes the pin 29. To hold the shieldplates 45 in position the base plate 41 is provided with upwardlyextending arms 49 which are bent over the upper edge of the pan 28 toform hooks 50 (Fig. 3).

The upper shield plates 46 are placed between a top plate 5| havingdownwardly extending arms 52 and a bottom plate 53, the latter havingupwardly extending arms 54 which are bent over the edge of the top plate5| to form hooks 55. To hold the assembly of shield plates 46 inposition above the pan 28 and spaced upwardly thereof a suitabledistance to permit the flow of vapor therefrom the downwardly extendingarms 52 of the top plate 5! are formed straight, and of proper length torest upon the bottom surface of the pan when the'upper shield plates 46are in operating position.

The shield plates 45 and 46 are spaced a short distance axially of eachother. If formed of metal the plates may be spaced by forming shortprojections 56 thereon, as best shown in Fig. 5, by a suitable stampingor like operation.

In a typical practical example of the system described herein inaccordance with my present invention for forming a deposit byvaporization, during a given period of operation of the system 2.5 gramsof bismuth constituting the charge placed in the pan 2! is required tobe evaporated, and deposited upon the surfaces of objects such as flatrectifier elements spaced over a spherical interior surface, such asthat of support I 8, which is two feet in diameter. Bismuth has areasonably high vapor pressure at 800 C. The metal parts of theevaporator in are preferably constructed of molybdenum since it is notattacked by either liquid or vaporous bismuth at the temperatures abovementioned.

The pan or holder 28 may be approximately 0.75 inch in diameter sincethe 2.5 gram charge of melted bismuth is adequately held in a containerof this size. The heating element 3!, of coiled filament form. is soarranged as to be contained within a space, between pan 28 and sphericalshell 23, .of one inch internal diameter and one and one-half inchoutside diameter, and

the shell element 23 is therefore of one and three surface andconsequent,

quarters inch minimum diameter in order adequ'ately to house the heatingelement 3| without danger of contact between the two metallic elements.

Since the spherical shell 23 is required to be maintained atapproximately 1100 C. and the outside area of the shell is 9.4 sq. in.,a power input to the shell of 230 watts is required, which is suppliedby the heater element 3|. If the heater element were assumed to beconstituted by an unenclosed straight wire at a temperature of 1300 C.,the required power would be supplied if the area of the wire were 4.6square inches. However, the space within shell 23 actually availableforthe heater 3| permits the employing of 60 inches of 0.033 inch diameterwire, having'a surface area of approximately six square inches, which isin practice suihcient to prevent the temperature of the wire, whenradiating 230 watts, from rising above 1300 (3., even though the wire isin coil form and is surrounded by the shell 23. Further, thistemperature, 1300 C., is sufficiently low to insure that the molybdenumwill not be attacked by the bismuth vapor. v

Since the diameter of the pan 28 is 0.75 inch and since the combinedarea of the 92 perforations 24 is preferably one-ninth of the area ofthe pan, the diameter of each perforation is approximately 0.026 inch.

The determining of a suitable number and thickness of the spaced platesconstituting the heat shielding elements 45 and 46 is in a. largemeasure a matter of trial and error. For use in connection with theevaporator 22 having the 0.75 inch diameter pan 28 evaporating 2.5 gramsof bismuth at each charge in the 1.75 inch diameter shell 23, theassemblies 45 and 46 each comprise eight circular sheets of molybdenumapproximately 0.001 inch in thickness.

The typical practical apparatus in accordance with my invention,constructed and operated as hereinabove described, produced upon theobjects spaced over the interior surface of the spherical support, adeposit of uniform thickness and having a satisfactorily high degree ofuniformity of physical nature throughout the extent of the deposit onthe several objects.

heater wire As illustrated in Figs. 6 and '7, which represent anembodiment of the invention wherein the vapor is to be projected fromperforations in an upper hemisphere only of an evaporator, I prefer todispense entirely with that portion of the evaporator not provided withperforations. In Figs. 6 and 'l the evaporator, as designated gen erallyby the numeral 51, then assumes the form of a hemispherical shell 58having perforations 59 therethrough, and having, instead of ahemispherical lower portion, a cover or base plate 60 removably hingedor fitted to the shell 58. Suitable pan or holder means for thesubstance to be evaporated and heater and heat shielding means for theholder maybe provided within the evaporator structure, essentially as inthe embodiment of the invention illustrated in Figs. 1 to 5.

The hemispherical shell 58 is provided with a vapor-reflecting or bafflemember constituted preferably by a collar or annular plate Si in closecontact with the shell 59 at its base or lower edge. The plate ii isarranged to be heated sufficiently, as by conduction from any suitableheating means preferably within the evaporator, to prevent condensation,on the latter plate, of vapor projected from the perforations in theshell 59. The heated annular plate 6| may extend out from the lower edgeof shell 59 nearly to the interior wall of the support, as ill of Fig.1, upon which the objects to be coated are mounted. However, in practiceit has been found that the plate 6|, to ensure substantial uniformity ofthe condensed deposits, need extend out only approximately onethird ofthis distance.

My invention has been described herein in particular embodiments forpurposes of illustration.

, It is to be understood, however, that the inven- The evaporatorstructure 22 as hereinabove described is capable of and is illustratedas arranged for the projecting of the vapor from the substance in pan 28in substantially all directions, so that rectifier carrier electrodes I:or like objects may be placed in all parts of the spherical support l0,and, when so placed throughout the support, will receive a uniformcondensed layer of the evaporated substance. In certain applications ofmy invention it is desired to project the vapor toward only a limitedportion of a holder such as spherical support Ill, thereby to form adeposit upon electrodes or like objects located only in this limitedportion of the holder. If the evaporated substance is to be depositedover objects on a hemispherical surface only, as for example overobjects located only on the upper hemisphere of spherical support l0,then perforations, .of suitable diameter, number and spacing may beformed only in the upper half of the,evaporator shell, omitting theperforations in the lower half to avoid using more of the evaporatedsubstance than necessary. A vapor-reflecting or baiiie means may beprovided associated with the evaporator shell to prevent more completelythe molecular rays from being projected uselessly into the lower andunused portion of the spherical support.

tion is susceptible of various changes and modifications and that by theappended claims I intend to cover any such modifications as fall withinthe true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. An apparatus comprising the combination of an evaporator shell, atleast a portion of which has the general shape of a portion of a sphereand having perforations therein, a holder within said shell adapted tocontain a substance to be vaporized, heating means operativelyassociated with said shell and said holder for projecting vapor of suchsubstance through said perforations, and means for supporting objectshaving surfaces to be coated in such relation to the vapor-emittingperforations of said evaporator that the surfaces to be coated aresubstantially equidistant from said vapor-emitting perforations.

2. A condensing apparatus comprising the combination of a sphericalsupport for objects upon which vapor is to be condensed, an evaporatortherein comprising a shell, a holder within said shell adapted tocontain a substance, and heating means within said shell for evaporatingsaid substance, said shell having perforations formed therein for theprojecting of the vapor of said substance therefrom, the total area ofsaid perforations being small relative to the area of said substance tobe evaporated.

3. A condenser apparatus comprising a spherical support, an evaporatorenclosed therein comprising a shell, a holder within said shell adaptedto contain a substance to be vaporized and means for vaporizing saidsubstance, said shell containing perforations which are internallycountersunk to facilitate outward flow of vapor therethrough.

4. A condenser apparatus comprising the combination of a sphericalholder, an evaporator located therein comprising a spherical shell, aholder therewithin for containing a vaporizable substance, and heatingmeans for vaporizing said substance, said shell having relatively smallperforations of the order of ninety two in number formed therein andspaced substantially uniformly thereover for projecting the vapor ofsaid substance uniformly exteriorly of said shell.

5. An apparatus for depositing vaporizable material on receivingsurfaces comprising a perforated spherical shell, a holder mountedwithin said shell adapted to contain a substance to be vaporized, aheating element located in the space between said shell and said holder,a spherical support for objects to be coated surrounding said shell andbeing spaced away from and substantially parallel to the exteriorsurface of said perforated shell.

6. An evaporator comprising a perforated shell, a holder spacedcentrally of said shell containing a substance to be vaporized, aheating element located in the space between said shell and said holder,a frame within said shell in rigid connection therewith, means to mountsaid heating means insulatingly on said frame, and a support for objectto receive deposits symmetrically located within vapor-depositingdistance external to said evaporator.

7. An evaporator comprising a perforated but otherwise closed shell, aholder within said shell for containing a substance to be vaporized,heating means in said shell, and radiation shielding means locatedbetween said holder and said heating means.

8. An evaporator comprising a shell, a, holder within said shellcontaining a substance, a heater enclosed within said shell, and aplurality of relatively thin plates interposed between said heater andsaid holder for retarding the rate of heat flow into saidsubstance fromsaid heater.

9. An evaporator comprising a shell having perforations therein, aholder within said shell containing a substance to be evaporated,heating means between said shell and said holder, and means forretarding the rate of heat flow from said heating means to saidsubstance comprising two groups of radiation shielding elements disposedon opposite sides of said holder and spaced from each other to permitescape of the vapor of said substance to said perforations.

10. The combination of an evaporator comprising a shell, a portion atleast of said shell having the general form of a portion of a sphere,said shell having perforations therein for the projecting of vaportherefrom, a holder within said shell adapted to contain a substance tobe vaporized, heating means within said shell to vaporize said substancethereby to Project said vapor through said perforations and means forsupporting spaced equidistant about said evaporator a plurality ofobjects in unimpeded vaper-receiving position.

11. An apparatus for coating the surfaces of a plurality of objects witha deposit comprising a housing constituting part of a sphere, bracketsfor attaching the objects to be coated to the interior surface of saidhousing, an evaporator having a, spherical shell concentric with saidinterior surface, and means within said evaporator shell for forming thevapor of said substance, said shell having a plurality of perforationsformed therein for the projection of said vapor radially toward saidsurfaces of the objects to be coated.

12. An evaporator comprising a hollow shell having a portion generallyhemispherical in form, said shell having perforations only in saidportion thereof for the projection of vapor from the interior of saidevaporator, a vapor-reflecting member operatively associated with saidshell portion for preventing said vapor from being projected inundesired directions from said evaporator, and a support for articles tobe coated which includes a hemispherical enclosure for said evaporatorwhich is spaced away therefrom, said enclosure being substantiallyconcentric with the hemispherical vapor-projecting shell of saidevaporator.

13. An evaporator comprising a hemispherical shell having perforationstherethrough for the projection of vapor from the interior of saidevaporator, a baille plate in contact with and surrounding the base ofsaid shell and extending a substantial distance from said shell radiallythereof and a support for articles to be coated which includes ahemispherical enclosure for said evaporator and is spaced awaytherefrom, said enclosure being substantially concentric with thehemispherical, vapor-projecting shell of said evaporator.

14. The combination of means for supporting a plurality of objects insuch relation that surfaces to be coated shall constitute a compositesurface having substantially the configuration of a segment of a sphere,an evaporator comprising a perforated hollow body having a surfaceconfiguration which is symmetrical with respect to said compositesurface and which is symmetrically spaced from said objects, and meansenclosed by said hollow body .for projecting vapor of a chosen coatingmaterial through the perforations of said evaporator upon said surfaces.

15. An apparatus for producing coatings by condensation of vaporcomprising a perforated, substantially spherical evaporator shell, aholder spaced centrally of said shell containing a substance to bevaporized, a heating element located in the space between saidevaporator shell and said holder, a frame within said shell in rigidconnection therewith, means to mount said heating means insulatingly onsaid frame, substan tially spherical support for articles to be coated,said support being spaced away from and substantially parallel to theexterior surface of said evaporator and a sealed envelope which isadapted to be evacuated enclosing said elements.

CLARENCE W. HEWLETT.

